This invention relates to strip transmission lines, and pertains particularly to a stripline circuit having two electromagnetically-decoupled striplines.
Stripline transmission lines are employed at microwave frequencies or frequencies involved in conventional radar systems to guide RF (radio frequency) energy. They also may be used as resonant circuits, impedance matching or measuring devices, or as the basic structures for various components in integrated-circuit design, and they frequently employ a configuration comprising a strip conductor sandwiched within a dielectric between two conductive groundplanes.
Although advantageous for miniature and subminiature designs, striplines have a relative disadvantage compared to some other transmission lines, such as coaxial cable and wave guides, in that unshielded and fringing RF fields more easily couple signals to other adjacent circuits. That is, a current flowing in a stripline causes an electromagnetic field to propagate toward other adjacent circuits without the field being confined, as it is in coaxial cable and waveguide devices, such that unwanted coupling results.
Existing stripline circuitboards employ various isolation techniques to combat this problem. For example, parallel outer groundplanes are often employed, with the stripline sandwiched within a dielectric between the outer groundplanes. This serves to confine the field. A center groundplane may be included in the plane of the stripline circuits, with the center groundplane and outer groundplanes being interconnected by plated-through holes or by Z-wires (wires bent in the general shape of the letter "Z"). This further confines the field. In addition, individual circuits are often carefully positioned relative to one another and to the center groundplane in an empirical determination of optimum component placement, whereby the resulting field pattern minimizes coupling (the "cut and try" method).
However, these techniques suffer certain drawbacks. They are often uncertain, time consuming, and of only limited success in adequately decoupling adjacent circuits.
Therefore, it is desirable to have a stripline circuit with improved electromagnetic decoupling. It is desirable that the decoupled circuit lend itself to standard circuitboard fabrication techniques without introducing significant additional expense. Additionally, it would be handy to have a method of refining the circuit layout once a circuitboard is fabricated in order to fine tune the design for minimum coupling.